1. Field of the Invention
The present invention relates to a cathode material for a lithium (Li) ion secondary battery having excellent capacity and output characteristics and safety, and to a Li ion secondary battery using the same.
2. Description of the Related Art
In order to adopt a Li ion secondary battery as a battery for hybrid cars, it is necessary to achieve reduction in cost, volume, and weight of the battery, while maintaining high safety. Thus, a cathode material is required to have high capacity, high output, and high safety.
Japanese Unexamined Patent Publication No. 2002-42890 discloses that lithium nickel oxide particles have the surfaces coated with particles of lithium cobalt oxide or lithium manganese oxide having a low surface catalytic function, whereby the lithium nickel oxide has the reduced surface catalytic function and thus is stabilized.
In the application of this technique to the batteries for the hybrid cars, the stability of the oxides is high, and also the safety of the cathode material is high. However, the surfaces of the lithium nickel oxide particles are coated, which may lead to an increase in resistance of the battery, resulting in insufficient output characteristics thereof.
On the other hand, Japanese Unexamined Patent Publication No. 2002-319398 discloses that mixing of a lithium cobalt composite oxide containing different kinds of elements and another lithium nickel composite oxide containing different kinds of elements prevents oxidation and dissolution of substitution elements by electron exchange between these two materials. The application of the technique to the battery for a hybrid car suppresses the dissolution of transition metal from the oxide, and thus improves cycle characteristics. However, the problem of a cathode material with a high content of Ni, that is, instability of a structure of the cathode material in charging is caused to release oxygen from the structure, which cannot be suppressed. As a result, when the internal temperature of the battery is increased due to internal short circuit or the like, the possibility of ignition becomes high. The formation of fine primary particles by a granulation method or the like does not appear in the lithium cobalt oxide, so that an adequate contact area between an active material and an electrolyte cannot surely be obtained, resulting in insufficient output characteristics. Furthermore, since the cobalt content of the transition metal elements in the lithium cobalt composite oxide of the cathode material is high, which makes it difficult to adopt the cathode material in the battery for the hybrid car.
Japanese Unexamined Patent Publication 2007-48711 discloses that particles of a cathode active material are coated with a stable composite oxide containing at least one of nickel and manganese thereby to improve chemical stability. In the application of the technique to the battery for the hybrid, the stability of the oxide is high, and also the safety of the cathode material is high. However, the surfaces of the oxide particles are coated, which may lead to an increase in resistance of the battery, resulting in insufficient output characteristics thereof.
In order to adopt a Li ion secondary battery as the battery for a hybrid car, the Li ion secondary battery is required to have high capacity, high output, and high safety. These characteristics are closely related to the properties of a cathode material. In the layered cathode material represented by the composition formula of LiMO2 (M: transition metal), a Ni content on the transition metal site needs to be increased so as to improve the capacity characteristics.
The inventors focus their attention on a mixture of two kinds of cathode materials with different properties so as to simultaneously achieve the high capacity, high output, and high safety.
The features of the invention will be described below.
According to a first aspect of the invention, a cathode material includes a first cathode active material represented by the composition formula: Lix1Nia1Mnb1Coc1O2, where 0.2≦x1≦1.2, 0.6≦a1, 0.05≦b1≦0.3, 0.05≦c1≦0.3, and having an average primary particle size of not less than 1 μm nor more than 3 μm, and a second cathode active material represented by the composition formula: Lix2Nia2Mnb2Coc2O2, where 0.2≦x2≦1.2, a2≦0.5, 0.05≦b2≦0.5, 0.05≦c2≦0.5, and having an average primary particle size of not less than 0.05 μm nor more than 0.5 μm.
An average secondary particle size of the first cathode active material is not less than 5 μm nor more than 30 μm, and an average secondary particle size of the second cathode active material is not less than 2 μm nor more than 10 μm. The average secondary particle size of the first cathode active material is 1.5 times or more as large as that of the second cathode active material.
A content of the first cathode active material in the cathode material is not less than 15% nor more than 70% in terms of percentage by mass.
A difference in Ni content between the first cathode active material and the second cathode active material is equal to or more than 20% in terms of atomic percentage.
According to a second aspect of the invention, a lithium ion secondary battery is provided which includes a cathode for absorbing and releasing lithium and an anode for absorbing and releasing lithium formed with an electrolyte positioned therebetween. The cathode includes a cathode active material, a cathode mixture containing a carbon-based conductive agent and a binder, and a current collector. The cathode active material includes two kinds of lithium oxides with different Ni contents, each containing Li, Ni, Mn, and Co, and a difference in Ni content between the cathode active materials is equal to or more than 20% in terms of atomic percentage.
The cathode active material includes a first cathode active material and a second cathode active material. A Ni content of the first cathode active material is larger than that of the second cathode active material, and an average primary particle size of the first cathode active material is larger than that of the second cathode active material.
The first cathode active material is represented by the formula: Lix1Nia1Mnb1Coc1O2, where 0.2≦x1≦1.2, 0.6≦a1, 0.05≦b1≦0.3, 0.05≦c1≦0.3, and the second cathode active material is represented by the formula: Lix2Nia2Mnb2Coc2O2, where 0.2≦x2≦1.2, a2≦0.5, 0.05≦b2≦0.5, 0.05≦c2≦0.5.
According to a third aspect of the invention, a lithium ion secondary battery is provided which includes a cathode for absorbing and releasing lithium and an anode for absorbing and releasing lithium formed with an electrolyte positioned therebetween. The cathode includes a cathode active material, a cathode mixture containing a carbon-based conductive agent and a binder, and a current collector. The cathode active material which includes lithium oxides containing Li, Ni, Mn, and Co includes a first cathode active material and a second cathode active material. A Ni content of the first cathode active material is larger than that of the second cathode active material, and an average primary particle size of the first cathode active material is larger than that of the second cathode active material.
An average secondary particle size of the first cathode active material is not less than 5 μm nor more than 30 μm, and an average secondary particle size of the second cathode active material is not less than 2 μm nor more than 10 μm. The average secondary particle size of the first cathode active material is 1.5 times or more as large as that of the second cathode active material.
The invention can provide a cathode material for a lithium ion secondary battery with excellent capacity and output characteristics and safety, and a lithium ion secondary battery with the excellent characteristics.